1. The Field of the Invention
The present invention relates to a method of making a cooking stove top, which can be equipped with at least two different glass ceramic tops with different IR spectral transmittance (type A, type B), which each have at least one cooking area, which is heated from below by means of a radiant heating body cooperating with a temperature-limiting adjusting device, which limits the surface temperature of the stove top installed in the cooking stove to a maximum value.
2. Related Art
Cooking stoves with a glass ceramic plate providing a cooking surface, a so-called stove top, have been on the market for many years in different forms. These systems are embodied as built-in cooking units, as table-top-cooking units, or as free standing cooking ranges. They typically have several cooking areas, also several cooking locations.
The radiant heating bodies used as heaters for these stove tops are predominantly electrical devices. The energy transfer from the source of the heat through the glass ceramic top to the cooking vessel bottom occurs by heat radiation. The resistor wire of the radiant heating bodies is heated up to a temperature of about 1300 K. One part of the emitted heat radiation from the heating coil goes directly through the glass ceramic top into the cooking vessel bottom or the surroundings in the vicinity of a cooking area when there is no cooking vessel on the cooking area. The remaining part of the emitted heat radiation is absorbed in the glass ceramic and heats the glass ceramic in the vicinity of the cooking area.
Halogen heating bodies are also used in a small percentage of the cooking stove tops. Halogen heating bodies operate according to the same principle as conventional radiant heating bodies. Generally in this case the heating coil is arranged in a quartz tube surrounded by a protective gas. The protective gas prevents contact of the heating coil with oxygen so that it is possible for the heating coil to operate at temperatures up to about 2400 K.
Different types of heating coils produce different radiation spectra, since they operate at different temperatures. The portion of the heat radiation that goes directly though the glass ceramic top and the portion that heats the cooking area are also different for the different types of heating bodies. Also different fractions of the input energy are delivered as primary heat radiation that passes directly through the glass ceramic and/or is absorbed by the glass ceramic and as secondary radiation to the surroundings at the same nominal power.
Further development of cooking apparatus has resulted in the use of the currently different glass ceramic types with different transmission properties. Because of required product differentiation current cooking apparatus manufacturers use different types of glass ceramic material in one and the same base structure or top structure using the “Panel Forming Engineering” techniques known from the automobile industry. The different radiant heating bodies must be individually adjusted to the different glass ceramics in order to maintain the existing safety standards and the desired minimum cooking times because of the different transmission properties of the different glass ceramic materials and the different emissivities of the different radiant heating bodies. The adjustment of the heating body output occurs by means of a temperature-limiting device, which limits the surface temperature of the cooking surface to a maximum value. This limiting device is required in order to protect the glass ceramic cooking surface and the cooking surface surroundings from overheating by limiting them to a maximum allowable temperature. It is especially important in the case of a built-in kitchen to limit the rear wall and side walls of the kitchen fittings along one wall of a kitchen to a maximum allowable temperature. The specifications for the surrounding temperatures at the rear wall and side walls of the kitchen fittings along one wall of a kitchen are given by safety standard described in EN 60335. The EN 60335, part 1, section 19, describes a test for determining the rear wall and side wall temperatures of the kitchen fittings arranged along one kitchen wall. A maximum temperature increase of 150 K is permitted during this test. The limiting temperature values of the individual heating bodies are determined by the ability of the glass ceramic used in the cooking unit to withstand high temperatures, but of course can also be determined by the temperature limits for the rear wall or side walls of the kitchen fittings.
These problems encountered when different glass ceramics are used currently may be practically solved only by separately storing the heating bodies for the different types of glass ceramics, which is contrary to the use of “Top or Plate Forming Engineering” methods and means high storage and logistics expenses.
To maintain and/or reduce the surrounding temperature of the rear wall and side walls of the cooking stove top DE 10 2004 023 847 A1 teaches formation of the underside of the glass ceramic cook top with lenses or prisms so that the primary heat radiation from the heating coil passing through the cooking area remains more focused in the vicinity of the cooking area and thus the temperature increase of the walls is reduced. The focusing of the primary heat radiation should occur by complex structuring of the glass ceramic underside. For example, structuring the glass ceramic top underside in the form of a Fresnel lens in the vicinity of the cooking area is proposed. Furthermore a sort of parallel prism structure is described, which of course can only act in one direction, either away from the rear wall or away from the side walls, but not away from both. However current cooking stove tops are currently made in a number of different outer geometries and equipped with different heating bodies. The selection of the heating body size and arrangement is currently almost completely arbitrary. The proposed formation of the cooking surface underside is not possible according to the current state of the art, since respective individual shaping rollers would be required for shaping the hot glass sheet of green glass to be ceramicized for each heating body and the product could not be further modified in subsequent processing steps. An additional disadvantage of a full-surface structuring of the underside of a glass ceramic cook top, for example with prisms or Fresnel lenses, is that a desired transparency of part of the cooking surface for display devices or the like is not possible.